Demonstration III: open zircon data
On fully open IsoplotR datasets the screening steps run end-to-end. A ²⁰⁴Pb-bearing suite is 67% discordant before correction (²⁰⁶Pb/²³⁸U ~429 Ma vs ²⁰⁷Pb/²³⁵U ~1300 Ma) and concordant at ~312 Ma after a Stacey–Kramers common-Pb correction; a clean suite sits at ~249 Ma untouched. A detrital population spanning ~129–2994 Ma yields only a maximum depositional age.
On fully open IsoplotR data the screening steps are shown end-to-end. A ²⁰⁴Pb-bearing suite is 67% discordant before correction (²⁰⁶Pb/²³⁸U ~429 Ma vs ²⁰⁷Pb/²³⁵U ~1300 Ma) and concordant at ~312 Ma after a Stacey–Kramers common-Pb correction; a clean suite sits at ~249 Ma untouched. A detrital population spanning ~129–2994 Ma yields only a maximum depositional age from its youngest concordant grains.
To show the screening steps on fully open data, we used the standard geochronology toolbox IsoplotR’s real example datasets[6]. [F]
Common-Pb screening and correction. A real ²⁰⁴Pb-bearing suite is, before correction, severely discordant—²⁰⁶Pb/²³⁸U ~429 Ma vs ²⁰⁷Pb/²³⁵U ~1300 Ma, a 67% discordance—because non-radiogenic common ²⁰⁶‧²⁰⁷Pb inflates the signal. This is the direct, empirical answer to the question “is there ²⁰⁶Pb in nature?”: there is, and it is measurable through ²⁰⁴Pb. Correcting for it (Stacey–Kramers common-Pb composition[7]) restores concordance at ~312 Ma. A second, clean suite is concordant at ~249 Ma without correction. [F]
Honest caveat. The common-Pb correction depends on an assumed common-Pb isotopic composition. Its leverage grows as the common fraction grows—i.e. for young, low-radiogenic grains—which is precisely why young U–Pb ages are intrinsically harder, independent of instrument quality (§11). [A]
Inheritance in detrital populations. In a real detrital-zircon sample the single-grain ages span ~129–2994 Ma and are dominated by old source grains. The depositional event is constrained only as younger than the youngest coherent grains (here ~130–200 Ma)—a maximum depositional age, never the deposition age itself. [F] This is the inheritance mechanism in its purest form, with the §7 safeguard that one screens by concordance first, because Pb loss can make a grain spuriously young.
